1. Field of the Invention
The present invention relates to an anchoring element for permanent anchorage of extra oral prostheses, such as bone anchored hearing aid devices or bone anchored ear or orbital prostheses in the skull bone. The invention is specifically intended to be used in connection with hearing aid devices of the bone conduction type, i e hearing aid devices by which the sound is mechanically transmitted via the skull bone directly to the inner ear of a person with impaired hearing. However, the invention is not limited to this specific application, but can be used in connection with other types of hearing aid devices for anchorage in the skull bone and for ear or orbital prostheses which are also anchored in the skull bone and also for finger prostheses or the like, i e in applications where the bone is forming a relatively thin layer.
2. Related Art
For persons who cannot or do not want to benefit from traditional, air conduction hearing aids there are other types of sound transmitting hearing aids on the market, i e bone anchored hearing aids which mechanically transmit the sound information to a persons inner ear via the skull bone by means of a vibrator. The hearing aid device is connected to an anchoring element in the form of an implanted titanium screw installed in the bone behind the external ear and the sound is transmitted via the skull bone to the cochlea (inner ear), i.e. the hearing aid works irrespective of a disease or damage in the middle ear or not. Penetration of the skin makes the vibratory transmission very efficient.
This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of impaired hearing, but also as anti-stuttering means. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling, a snap in coupling, magnetic coupling or the like. One example of this type of hearing aid device is described in U.S. Pat. No. 4,498,461. Another example is the Baha® bone anchored hearing aid marketed by Cochlear Bone Anchored Solution AB (previously Entific Medical Systems AB) in Goteborg, Sweden. In WO 2005/037153 it is described how this type of hearing aid device can be used as anti stuttering means.
Fixtures used so far for bone anchored hearing aid devices of the type which have been mentioned here as well as for existing ear or orbital prostheses, have been designed in such a way that a screw tap has been required to form an internal thread in the hole drilled in the skull bone or they have been self-tapping. One example of the first mentioned type of fixture is illustrated in US Des. 294,295. This fixture has an external thread with small cutting edges with only a minor scraping effect in the pre-tapped bone hole. It has also a flange which functions as a stop against the bone surface when the fixture is screwed down into the skull bone. The flange is also in this case provided with through holes for bone ingrowth or the like.
Self-tapping fixtures for bone anchored hearing aid devices have been described in for instance U.S. Pat. Nos. 7,074,222 and 7,116,794. These fixtures can normally be installed in the skull bone without the use of any screw taps. This means that the installation can be carried out in a more simple way.
As the skull bone is relatively thin, that part of the anchoring element which is intended to be inserted into the skull bone is normally shorter than 5 mm, and it is important that the anchoring element is provided with a flange which functions as a stop when the fixture is installed into the thin skull bone. The fixtures are normally made of titanium which material has a known ability to be integrated into the surrounding bone tissue, so-called osseointegration.
It is expected that an increased contact surface between the anchoring element and the surrounding bone tissue would be beneficial for a good and stable anchorage. In other implant fields, for instance in the dental implant field, the length of the fixtures can be increased in order to increase the contact surface to the surrounding jaw bone tissue. However, the possibility to increase the length cannot be used for fixtures intended to be installed in the thin skull bone. As already mentioned the fixture lengths are limited to approximately 5 mm in this case. Instead of length, the thickness of the fixture screw could be increased to some extent. The diameter of a fixture used for a bone anchored hearing aid device is typically within the range of 3.5-5 mm, compared to 3-4 mm for a dental implant fixture. To increase the fixture screw diameter even more would not be practical as such a fixture would be too difficult to install.